by Paul Smart | Updated: 04/07/2016 | Comments: 24
Did you know that you can turn your Campbell Scientific data logger into a Modbus TCP/IP server? Do you know why that’s a good idea? Campbell Scientific data loggers are commonly used as Modbus server devices. This means that the data logger is configured to implement the Modbus communications protocol and listen for Modbus polls from a Modbus client, such as a SCADA (supervisory control and data acquisition) system. Turning your data logger into a Modbus TCP/IP server is a great way to allow systems that use the Modbus protocol to have access to your live measurement data.
To better understand how this works, we’ll go through an example exercise to show the basic concepts of how to implement the Modbus protocol on a CR1000 datalogger. We’ll use a CR1000 datalogger connected to a SCADA system. You may remember our previous discussion about SCADA systems and Modbus from the “Why Modbus Matters: An Introduction” blog article.
In our example, the CR1000 is programmed to measure battery voltage, panel temperature, and an analog measurement in a simple program as shown below:
'Program for a CR1000 Series Datalogger 'Declare Public Variables Public PTemp, batt_volt, analog_meas 'Data Table Definition DataTable (Table1,1,-1) DataInterval (0,10,Min,10) Minimum (1,batt_volt,FP2,0,False) Average (1,PTemp,FP2,False) Average (1,analog_meas,FP2,False) EndTable 'Main Program BeginProg Scan (1,Sec,0,0) 'Measure the datalogger panel temperature PanelTemp (PTemp,250) 'Measure the battery voltage Battery (batt_volt) 'Measure an analog voltage VoltSe (analog_meas,1,mV5000,1,1,0,_60Hz,1.0,0) 'Call final storage table CallTable Table1 NextScan EndProg
The CR1000 datalogger in our example is connected to a SCADA system using an NL121 Ethernet Interface. Therefore, we need to program our data logger to listen to Modbus polls on the appropriate communications port and to respond with our most current measured data. To do this, we need to use the ModbusServer() instruction, declare a variable array to hold our Modbus data, and then update that array with our measurements.
This is accomplished using the code below:
'Program for a CR1000 Series Datalogger 'Declare Public Variables Public PTemp, batt_volt, analog_meas Public ModbusRegisters(3) Public ModbusCoil As Boolean 'Data Table Definition DataTable (Table1,1,-1) DataInterval (0,10,Min,10) Minimum (1,batt_volt,FP2,0,False) Average (1,PTemp,FP2,False) Average (1,analog_meas,FP2,False) EndTable 'Main Program BeginProg 'Configure the datalogger as a Modbus Server ModbusServer (502,0,1,ModbusRegisters(),ModbusCoil,2) Scan (1,Sec,0,0) 'Measure the datalogger panel temperature PanelTemp (PTemp,250) 'Measure the battery voltage Battery (batt_volt) 'Measure an analog voltage VoltSe (analog_meas,1,mV5000,1,1,0,_60Hz,1.0,0) 'Populate Modbus Registers ModbusRegisters(1) = PTemp ModbusRegisters(2) = batt_volt ModbusRegisters(3) = analog_meas 'Call final storage table CallTable Table1 NextScan EndProg
Let’s take a closer look at the ModbusServer() instruction we added to the program code. The instruction is added between the BeginProg and Scan statements. The ModbusServer() instruction is placed between these two statements because it only needs to execute once at compile time rather than being executed during each scan.
The ModbusServer() instruction contains six parameters as shown below:
ModbusServer (COMPort,BaudRate,ModbusAddr,ModbusVariable,BooleanVar,ModbusOption)
For reference, in our program code, our instruction with the parameters looks like this:
ModbusServer (502,0,1,ModbusRegisters(),ModbusCoil,2)
Let’s take a closer look at those parameters and what they mean:
Modbus Registers | Measurement Description | Units |
1,2 |
Data Logger Panel Temperature |
Degrees Celsius |
3,4 |
Data Logger Battery Voltage |
Volts |
5,6 |
Analog Measurement |
Millivolts |
The example exercise above outlines how you can accomplish a simple Modbus TCP/IP server implementation by programming a CR1000 datalogger in CRBasic. Please note that the Short Cut program generator also has the capability to generate the code we have discussed above. By taking advantage of the concepts shown above, you can add more measurements to your program, as well as to the Modbus Register Map of your data logger. The result is that your SCADA system can access live measurement data for a more complete weather station implementation.
You can look forward to blog articles in the future with more details on specific Modbus topics, common pitfalls, and best practices. In the meantime, share any Modbus comments or questions you have below.
Comments
raichlebw | 10/05/2016 at 05:41 PM
Hi,
I've been trying to set up a Modbus TCP server on a CR1000 with no success so far. I'm testing the server with an Obvius Acquisuite 8810 Modbus TCP client, which has not yet established communition with the logger/server. We're pretty good with Modbus here, but not as good with network communication.
The questions...
1) With the ComPort parameter of ModbusServer set to 502, to which physical port should the NL121 be attached? I assume it's the RS-232 port. It's interestinng that, whichever of the 2 possible answers (RS-232, CS I/O) is the case, there will be Modbus TCP transmitted over a serial cable.
2) With the ComPort parameter of ModbusServer set to ComRS232, is the CR1000 a Modbus RTU server? This must be the case to avoid ambiguity with the TCP server, but I've seen no mention of RTU in any Campbell literature.
Thanks,
Brian
Paul Smart | 10/06/2016 at 03:23 PM
Hi Brian,
Are you using an NL201 rather than an NL121? An NL121 will only connect to the peripheral port of the datalogger, whereas an NL201 can connect to the RS-232 port or the CSI/O port of the datalogger.
If you are using an NL201, then you will need to configure the NL201 either in bridge mode or as a Modbus TCP gateway. If you configure it as a Modbus TCP gateway, then your datalogger will need to use the appropriate port (RS232 or CSI/O) and will function as an RTU. If the NL201 is set up in bridge mode you will connect it to the CSI/O port of the datalogger and use port 502 in the ModbusServer instruction. RTU vs Modbus TCP/IP is determined by the port selection in the ModbusServer instruction. Make sure baud rates and ports in the ModbusServer instruction match the settings of the NL201.
I would recommend taking a look at the settings for the NL201 using Device Configuration Utility.
Please refer to the helps in Device Configuration Utility for information on the options available. The helps explain all of this a lot better than I just did :)
I hope this helps,
-Paul
ariklee | 02/23/2018 at 04:25 PM
Hi Paul, couple questions making CR1000x function as Modbus server over Ethernet:
1. Is port 502 set by the Modbus client? Should I verify with the client that they are indeed using 502, or can that not be changed?
2. Your ModbusCoil parameter is a single value boolean, while in Campbell's Application Note on Modbus Server, they use an 8-value boolean array. I'm a bit confused on this. (In my application, the client should not be sending any commands 01, 02, 05, or 15. Does it matter then?)
3. Can you recommended any modbus client simulators for Windows 10 (besides Modbus Poll)?
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